The present invention relates to a process for preparing high purity 2,2-bis(4-hydroxyphenyl)propane (hereinafter referred to as bisphenol A).
Bisphenol A is used as a raw material for polycarbonate resins or epoxy resins, and colorless and high purity bisphenol A is required for polycarbonate resin in particular.
Bisphenol A is prepared from acetone and excess phenol in the presence of an acidic catalyst, in some cases by the addition of an auxiliary catalyst such as sulfur compounds. The reaction mixture contains the catalyst, unreacted acetone, unreacted phenol, water formed by the reaction and other by-products of the reaction.
The by-products are mainly composed of 2-(2-hydroxyphenyl)2-(4-hydroxyphenyl)propane and also contain Dianin's compound, trisphenol, polyphenol and undesirable colored substances. These by-products deteriorate the properties of resin prepared from bisphenol A.
In a process for recovering high purity bisphenol A from the reaction mixture, the catalyst, water and a small amount of phenol are removed from the reaction mixture. The residual liquid mixture is cooled to crystallize the adduct of bisphenol A and phenol. The crystals are separated from the mother liquor containing the by-products of the reaction and then bisphenol A is recovered by removing phenol from the adduct.
As a process for removing phenol from the adduct of bisphenol A and phenol, various methods such as distillation, extraction, steam stripping etc. have been proposed.
For example, Japanese Patent Publication TOKKOSHO No. 52-42790 (1977) discloses a process for vaporizing the adduct at above 180.degree. C. for 0.1-30 minutes under reduced pressure and then obtaining bisphenol A by fractional condensation.
Japanese Patent Publication TOKKOSHO 36-23335 (1961) teaches a process for heating the adduct at 50.degree. C. or more by use of a solvent having a boiling point of at least 50.degree. C. and dissolving the phenol part alone in the solvent.
A distillation process is generally used because it is carried out with simple equipment and without other solvents or water. For example, Japanese Patent Publication TOKKOSHO No. 56-13700 (1981) describes a process for distilling off phenol under reduced pressure, then removing the decomposable low-boiling fractions by a fore-running removal column and successively obtaining bisphenol A by distillation. In addition, there are disclosed processes for obtaining bisphenol A, after removing phenol under reduced pressure, by conducting the distillation in the presence of stabilizers such as aliphatic dicarboxylic acid esters [Japanese Patent Publication TOKKOSHO No. 45-22539 (1970)], glycols [Japanese Patent Publication TOKKOSHO 45-3925 (1970)] and propylene glycol or epoxy resin [Japanese Patent Publication TOKKOSHO No. 56-1297 (1981)].
In the process of distilling the adduct of bisphenol A and phenol and obtaining phenol from the top of column and bisphenol A from the bottom respectively, adduct crystals or their fused liquid are required to be heated above a specific temperature prior to being fed into the distillation column. High temperatures are required for completely eliminating phenol. For example, Japanese Patent Publication TOKKOSHO No. 50-12428 (1975) teaches that desirable heating temperatures are at least 205.degree. C., e.g. the temperature at which phenol crystals are subjected to stripping.
The present inventors have found that bisphenol A solidifies in the distillation column and finally the operation becomes impossible when the temperature of adduct crystals or their fused liquid which are fed to the distillation column is lower than a specific temperature. Therefore the inventors have extensively investigated in order to overcome the above problem. Consequently, when the operating pressure is at least 50 Torr, it has been found that to prevent the solidification, the fused liquid of the adduct is fed to the distillation column at not less than 120.degree. C. or the bottom temperature of the column is maintained at not less than 180.degree. C. Under these conditions, however, substantial amounts of phenol remain so as to yield only substantially useless bisphenol A. On the other hand, when the operating pressure is less than 50 Torr, the solidification can be prevented by maintaining the temperature of the feed not less than 205.degree. C. Besides a column having a diameter of several centimeters such as used in the laboratory can be continuously operated by external heating, even though the feeding temperature or the bottom temperature is outside of above mentioned range. The external heating of the column, however, has been found not to be practical for industrial scale equipment.
On the other hand, treatment of bisphenol A at too high a temperature causes undesirable decomposition and the intact bisphenol A obtained from the bottom of column has been found useless as a product even though any of above process is employed.